The present invention relates to subterranean operations and, more particularly, to settable spotting compositions comprising cement kiln dust (“CKD”), and associated methods of use in subterranean operations.
During the drilling of a well bore in a subterranean formation, a drilling fluid may be used to, among other things, cool the drill bit, lubricate the rotating drill string to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to counteract the sudden entrance into the well bore of high pressure formation fluids, and remove drill cuttings from the well bore. A drilling fluid may be circulated downwardly through a drill pipe and drill bit and then upwardly through the well bore to the surface. The drilling fluid used may be any number of fluids (gaseous or liquid) and mixtures of fluids and solids (such as solid suspensions, mixtures, and emulsions).
Once the well bore has been drilled to a desired depth, the drill string and drill bit may be removed from the well bore and the drilling fluid may be left in the well bore to provide hydrostatic pressure on the formation penetrated by the well bore, e.g., to prevent the flow of formation fluids into the well bore. Next, a pipe string (e.g., casing, liners, etc.) may be introduced into the well bore. Depending on the depth of the well bore and whether or not any problems are encountered in introducing the pipe string into the well bore, the drilling fluid may remain relatively static in the well bore for a relatively long time period, for example, up to about 2 weeks or longer. While drilling fluids are generally not settable (e.g., they generally do not to form a hardened mass over time), drilling fluids may increase in gel strength over time. As referred to herein, “gel strength” is the measure of the ability of a colloidal dispersion to form a gel and is based upon its resistance to shear. Accordingly, during the time period that the drilling fluid is static in the well bore, portions of the drilling fluid may increase in gel strength so that displacement of the drilling fluid from within the well bore may be become more difficult.
After the pipe string has been introduced into the well bore, the pipe string may be cemented in the well bore by pumping a cement composition into an annular space between the walls of the well bore and the pipe string disposed therein, thereby displacing the drilling fluid in the annulus. However, if the drilling fluid has developed sufficient gel strength due to remaining static in the well bore, portions of the drilling fluid in the annulus may not be displaced. Since the drilling fluid generally is not settable, this may be problematic, among other reasons, because it may lead to the loss of zonal isolation, which can result in formation fluids continuing to enter and flow in the well bore due to the portions of the drilling fluid remaining therein.
Heretofore, settable spotting compositions have been used in the displacement of drilling fluids from well bores. As referred to herein, the term “settable spotting composition” refers to a fluid which over time sets to form a hardened mass, but will not set for a desired period of time, for example, from about 1 day to at least 2 weeks. In some instances, settable spotting compositions may be used to displace the drilling fluid from the well bore prior to the drilling fluid having gained a significant amount of gel strength, for example, prior to introducing the pipe string into the well bore. Generally, these settable spotting compositions should not have an undesirable increase in gel strength after being static in the well bore a period of time, for example, up to at least two weeks, so that the settable spotting compositions may be displaced from the well bore. After the well bore is at least partially filled with the settable spotting composition, the pipe string to be cemented may be introduced into the well bore. When the cement composition is pumped through the pipe string into the annulus, the drilling fluid (if any) and settable spotting composition in the pipe string and annulus should be displaced ahead of the cement composition. The settable spotting composition, if any, remaining in fractures or other permeable portions of the subterranean formation should set into a hardened mass, thereby preventing or reducing the entry or flow of formation fluids in the annulus.
Settable spotting compositions used heretofore, in some instances, have comprised blast furnace slag and other hydraulic components. To prevent a slag-containing settable spotting composition from prematurely setting, a very strong set retarder may be included therein and the settable spotting composition should be separated from the cement composition by a spacer fluid. If intermixing between the cement composition and the set retarded settable spotting composition occurs, the cement composition may be prevented from setting by the strong set retarder in the settable spotting composition. Another settable spotting composition that has been used, in some instances, to displace drilling fluids from well bores includes compositions that comprise water, fly ash, a gel strength inhibiting additive, and a set retarding additive. “Fly ash,” as that term is used herein, refers to the residue from the combustion of powdered or ground coal, wherein the fly ash carried by the flue gases may be recovered, for example, by electrostatic precipitation. Class F fly ash generally does not contain sufficient lime, so an additional source of calcium ions is required for the Class F fly ash to form a settable spotting composition with water.
Oil-based settable spotting compositions have also been used heretofore in the displacement of drilling fluids from well bores. For instance, a conventional settable spotting composition comprises an oil-external emulsion that comprises oil, water, an emulsifying surfactant, a hydraulically settable component, and optionally a demulsifying surfactant. Generally, the demulsifying surfactant may be included in the spotting compositions for demulsifying the oil-external emulsion when contacted by external water. The hydraulically settable component should be present in the oil phase of the oil-external emulsion to delay the setting of the settable spotting composition until after the demulsification of the oil-external emulsion.
During the manufacture of cement, a waste material commonly referred to as “CKD” is generated. “CKD,” as that term is used herein, refers to a partially calcined kiln feed that is removed from the gas stream and collected during the manufacture of cement. Usually, large quantities of CKD are collected in the production of cement, and they are commonly disposed of as waste. Disposal of the waste CKD can add undesirable costs to the manufacture of the cement, as well as the environmental concerns associated with its disposal. The chemical analysis of CKD from various cement manufactures varies depending on a number of factors, including the particular kiln feed, the efficiencies of the cement production operation, and the associated dust collection systems. CKD generally may comprise a variety of oxides, such as SiO2, Al2O3, Fe2O3, CaO, MgO, SO3, Na2O, and K2O.